This application claims the benefit of U.S. Provisional Application No. 60/339637 filed Dec. 12, 2001 entitled EFFICIENT AC/DC SOLID STATE SWITCHING SYSTEM FOR AIRCRAFT LANDING LIGHTS.
1. Field of the Invention
This invention relates to a switching and control system for an aircraft retractable landing light. Specifically, this invention relates to a switching and control system for an aircraft retractable landing light having solid state AC/DC switching and improved load control.
2. Description of the Related Art
Landing lights are mounted to aircraft to illuminate areas forward of the aircraft during night operations on taxiways and runways. A xe2x80x9cretractable landing lightxe2x80x9d is a remotely controlled, articulating light that can be stowed flush with the outside surface of the aircraft when not in use, reducing aerodynamic drag on the aircraft. The retractable landing light""s lamp is housed in a lighthead. The lighthead is in turn hingedly affixed to a housing assembly, which is mounted to the airframe. The lighthead is typically extended for use and retracted to a stowed position by means of a transmission mechanism driven by an electric motor, and held in place by an electromechanical brake. When the flight crew actuates a remote control to the xe2x80x9cExtendxe2x80x9d position, the brake is released by applying electrical power to the brake""s coil. Electrical power is simultaneously applied to the motor, causing the motor""s output shaft to turn, driving the transmission. The lighthead then extends to a predetermined position in conformance with the remote control, aiming the lamp to illuminate areas forward of the aircraft. Power is then removed from the motor and the brake, causing the lighthead to stop moving. The brake re-engages, holding the lighthead in position against the force of the windstream. The motor and brake are also simultaneously activated when the remote control is placed in the xe2x80x9cRetractxe2x80x9d position. However, power is applied to the motor so as to cause the motor""s output shaft to rotate counter to the direction used to extend the lighthead. Once the lighthead is flush with the surface of the aircraft, power is removed simultaneously from the motor and brake, holding the lighthead in the stowed position. The lamp may be automatically switched on by means of a limit switch after the lighthead is extended and then switched off when the lighthead is retracted. Alternatively, the lamp may be manually controlled by a switch in the cockpit.
Prior retractable landing lights suffer from a number of shortcomings. In particular, prior retractable landing lights utilize electromechanical relays for control of the motor, brake, and lamp. These relays have limited operational life due to wearing of the mechanical movement. In addition, the relays generate electromagnetic interference due to arcing at the relay contacts when switching inductive loads such as the motor and brake, and when switching loads with high inrush currents, such as incandescent lamps. This relay contact arcing also further reduces the operational life of the relays. Another disadvantage of electromechanical relays is that current flow through the motor, brake and lamp is limited only by the capacity of the aircraft""s electrical system and wiring. This places significant stress on the relay contacts, motor, brake, and lamp, causing these components to suffer reduced service life.
The action of the motor, brake, and lamp relays is controlled by mechanical limit switches and actuators in prior retractable landing lights. The limit switches and actuators make synchronization of motor, brake, and lamp functions cumbersome. As a consequence the motor and brake are switched on and off simultaneously. The lamp may also be switched on and off at the same time. This causes large, sudden load swings in the aircraft""s electrical system, resulting in electrical switching transients and electromagnetic interference that may disrupt or damage other equipment connected to the electrical system. Synchronization of the power applied to the motor, brake, and lamp is desirable to minimize switching transients and electromagnetic emissions. For example, the brake coil should be energized to release the brake prior to energizing the motor, and should be de-energized after removing power to the motor. The lamp should be switched on and off only when power has been removed from both the motor and the brake.
xe2x80x9cSoft startingxe2x80x9d of loads with high inrush current characteristics to extend component life is not new. For example, Hamilton U.S. Pat. No. 6,315,435, teaches the use of solid-state switches to reduce the inrush current to a lamp. However, Hamilton does not teach the use of synchronization and controlled turn-off of loads in addition to controlled turn-on. Hamilton also does not teach a solid state switch capable of switching either AC or DC electrical loads with galvanically isolated control logic. The use of optically coupled solid state switches for electrical power control is also well known, as shown in Rodriguez, U.S. Pat. Nos. Re. 35,836 and 4,390,790, Shichi et al. U.S. Pat. No. 5,612,582, Hodges U.S. Pat. No. 4,665,316, Pernyeszi U.S. Pat. No. 4,902,901, and McDonald U.S. Pat. No. 4,611,123. However, the prior art concentrates on achieving the faster turn-on and turn-off speeds desired for many switching applications rather than utilizing and enhancing the inherently slower switching characteristics of optically coupled field effect transistors to achieve synchronized, xe2x80x9csoftxe2x80x9d switching of multiple loads.
The landing light is a required item for night aircraft operations, and must be kept in good working order. Additionally, maintenance of the landing light is expensive and time-consuming. There is a need for a more reliable switching and control system for retractable landing lights.
This invention is directed to an efficient solid state switching and control system for aircraft retractable landing lights. The solid state switching and control system has xe2x80x9csoftxe2x80x9d turn-on and turn-off capability, hysteresis, and feedback to reduce stress on switching components, reduce electromagnetic interference, and extend the service life of the retractable landing light.
Specifically, the present invention includes solid state switches comprising back-to-back N-channel Metal Oxide Semiconductor Field Effect Transistors (xe2x80x9cMOSFETsxe2x80x9d) driven by photovoltaic optical drivers to provide galvanic isolation between the logic and power stages of the switching system. The combination of MOSFETS and photovoltaic optical drivers have an inherent xe2x80x9csoftxe2x80x9d switching characteristic, which provides for a slower turn-on and turn-off of the load than is possible with electromechanical relays. This characteristic is desirable, for example, for switching power to an incandescent lamp. The filaments of incandescent lamps exhibit a lower resistance when cool. As a result, when power is applied to an incandescent lamp, the lamp experiences an inrush of current that is much higher than its normal operating current, stressing the lamp""s filament and reducing its operational life. Soft-starting the incandescent lamp limits the inrush current, thereby increasing the lamp""s operational life. Controlled turn-off, coupled with soft starting, can also be beneficially applied to the retractable landing light""s motor and brake. A controlled turn-on and turn-off of the motor and motor brake acts to minimize the inductive energy generated by the motor and brake, resulting in improved component life and reduced electromagnetic emissions. If a cockpit-mounted indicator is employed to notify the flight crew when the lighthead is extended, soft-switching may also be used to extend the operational life of the indicator. Galvanic isolation of the logic stage of the present switching and control system facilitates control of either highlevel AC or high-level DC voltages, permitting the use of AC or DC motors, brakes, lamps, and indicators or a combination of AC and DC motors, brakes, lamps, and indicators. For example, it may be convenient to power the motor and brake from a relatively low-power DC aircraft electrical supply, but power the high-current lamp from a more robust AC aircraft electrical supply.
The solid state switches in the present invention may be coupled with feedback means, providing the logic stage with status information on the switched load. The logic circuit, acting in combination with the inherent soft-start and soft-stop characteristics of the solid state switches, provides hysteresis to stagger switching of the lamp, motor, and brake. This xe2x80x9csynchronized switchingxe2x80x9d reduces electromagnetic emissions, prevents shoot-through currents that can reduce the life of totem-pole switching components, and minimizes load-switching transients in the aircraft electrical system.
The present invention comprises a solid state switching system for a vehicle retractable light, comprising: means for controlling position of a lighthead and switching electrical power to a motor, motor brake, and lamp of the retractable light such that the power applied to said motor, motor brake, and lamp is gradually switched on and off; and solid state switches capable of switching high levels of AC or DC electrical voltage and current, said solid state switches comprising two N-Channel MOSFETs arranged in a series configuration such that a drain terminal of a first MOSFET is connected to electrical power, a drain terminal of a second MOSFET is connected to an electrical load to be switched, source terminals of said first and second MOSFETs are connected together, a photovoltaic generator is connected to gate terminals of said first and second MOSFETs, said photovoltaic generator providing a galvanically isolated current source to actuate said first and second MOSFETs when current is applied to light emitting diodes of said photovoltaic generator.